The present invention relates to a method and apparatus for polishing a semiconductor substrate whereby chemical mechanical polishing (CMP) is performed with respect to a semiconductor substrate of silicon or the like to flatten a surface thereof.
From the 1990s, CMP technology for polishing semiconductor substrates of silicon or the like has shown increasing tendencies toward single-wafer processing as the semiconductor substrates processed by CMP have had larger diameters on the order of 10 cm or more. Since design rules of 0.5 .mu.m or less have been used to form an extremely small pattern on a semiconductor substrate, equal polishing should be performed with respect to the entire surface thereof.
Below, a method and apparatus for polishing a semiconductor substrate according to a first conventional embodiment will be described with reference to the drawings.
FIG. 8 schematically shows the construction of the apparatus for polishing a semiconductor substrate according to the first conventional embodiment, in which is shown a platen 21 including: a substrate holder 21a made of a rigid material and having a flat surface; a rotary shaft 21b extending vertically downwardly from the back face of the above substrate holder 21a; and rotating means (not shown) for rotating the above rotary shaft 21b. To the top surface of the substrate holder 21a of the platen 21 is adhered an elastic polishing pad 22. Above the polishing pad 22 is provided a substrate holding head 24 which holds and rotates a semiconductor substrate 23. The semiconductor substrate 23 is rotated and pressed against a first region of the polishing pad 22 by the substrate holding head 24. A slurry 25 in a prescribed amount is supplied dropwise from an abrasive supply pipe 26 onto the polishing pad 22.
In the apparatus for polishing a semiconductor substrate thus constructed, the polishing pad 22 supplied with the slurry 25 is rotated by rotating the platen 21 and the semiconductor substrate 23 is pressed against the rotating polishing pad 22 by the substrate holding head 24 so that a surface of the semiconductor substrate 23 is polished under pressure at a relative speed.
In this process, if the surface of the semiconductor substrate 23 is rugged, the polishing rate is increased at projecting portions of the semiconductor substrate 23 since their contact pressure with the polishing pad 22 is high. On the other hand, the polishing rate is reduced at recessed portions of the semiconductor substrate 23 since their contact pressure with the polishing pad 22 is low. Consequently, the surface of the semiconductor substrate 23 becomes less rugged and more smooth.
However, the above method and apparatus for polishing a semiconductor substrate according to the first conventional embodiment present the following problems, which will be described with reference to FIGS. 9 to 11.
FIG. 9(a) qualitatively shows a relationship between the pressing period for the polishing pad 22 and the thickness of the polishing pad 22. In FIG. 9(a), the vertical axis represents the thickness of the polishing pad and the horizontal axis represents the pressing period for the polishing pad, which is the time elapsed since the initiation of polishing. FIG. 9(b) qualitatively shows the thickness of the polishing pad recovering with time after it is pressed under a given pressure subsequently reduced to 0. In FIG. 9(b), the vertical axis represents the thickness of the polishing pad and the horizontal axis represents the time elapsed since the pressure is reduced to 0 after pressing.
FIG. 10 is a schematic plan view illustrating different pressing periods for the polishing pad 22 depending on different radial positions of the polishing pad 22. FIG. 11 is a schematic cross-sectional view showing the construction of the platen 21 during polishing, which is taken along the diameter of the platen 21 (along the line VI--VI of FIG. 10).
When a commonly-used elastic material, such as one containing non-woven fabric or urethane foam as the main component, is used to compose the polishing pad 22, the amount of elastic deformation at the moment at which the polishing pad 22 initiates receiving pressure (within several seconds) differs depending on the time elapsed since the initiation of pressing, as shown in FIG. 9(a). When the polishing pad 22 is released from the pressure, the amount of elastic deformation is gradually reduced with the passage of time, as shown in FIG. 9(b).
According to the polishing method of the first conventional embodiment shown in FIG. 8, the period during which the polishing pad 22 is in contact with the semiconductor substrate 23 (i.e., the period during which the polishing pad 22 is pressed) varies. Specifically, the period during which the polishing pad 22 is in contact with and pressed by the semiconductor substrate 23 is different at the position a corresponding to the radius r1, at the position b corresponding to the radius r2, and at the position c corresponding to the radius r3 shown in FIG. 10. The period during which the polishing pad 22 is not pressed (the period for elastic deformation) is also different at the positions a, b, and c shown in FIG. 10. Consequently, on the polishing pad 22 in contact with the semiconductor substrate 23, the amount of elastic deformation of the polishing pad 22 is larger at the position b corresponding to the radius r2 substantially passing through the center of the semiconductor substrate 23 where the pressing period is relatively long and the recovering period is relatively short, while the amount of elastic deformation of the polishing pad 22 is smaller at the respective positions a and b corresponding to the radii r1 and r3 where the pressing period is relatively short and the recovering period is relatively long.
Accordingly, the thickness of the polishing pad 22 is smaller at the radially middle portion thereof (position b) and larger at the radially inner and outer portions thereof (positions a and c) when viewed in a cross section taken along a radius of the polishing pad 22 as shown in FIG. 11.
If such a polishing pad 22 is used to polish a flat semiconductor substrate 23 in contact with and pressed by the polishing pad 22, the polishing pad 22 exerts a higher pressure on the radially inner and outer portions of the polishing pad 22, while exerting a lower pressure on the radially middle portion of the polishing pad 22. A difference in pressure on the surface of the polishing pad 22 corresponds to a difference in polishing rate so that the polishing rate at the radially middle portion of the polishing pad 22 is lower than the polishing rate at the radially inner and outer portions of the polishing pad 22. Hence, it is impossible to equally polish the entire surface of the semiconductor substrate 23.
To overcome the problem, there has been proposed a technique utilizing the deformation of a semiconductor substrate 33 against the elastic deformation of the polishing pad 32, as shown in FIG. 12.
The following is the description of a method and apparatus for polishing a semiconductor substrate according to a second conventional embodiment. To a platen 31 is adhered an elastic polishing pad 32. The lower portion of a substrate holding head 34 for holding a semiconductor substrate 33 is provided with an airtight space 37 defined by a head main body 35 having a recessed portion and a plate-like elastic member 36 which can be elastically deformed and is provided in the recessed portion of the head main body 35. A gas under controlled pressure is introduced from a gas supply path 38 into the airtight space 37. Thus, by using the gas under pressure introduced into the airtight space 37 to press the semiconductor substrate 33 against the polishing pad 32 with intervention of the elastic member 36, equal polishing is performed by evenly pressing the semiconductor substrate 33 from the back face thereof.
However, in the case where there is a difference in the thickness or in the amount of elastic deformation of the polishing pad 32, the semiconductor substrate 33 is deformed (warped) to follow the rugged configuration of the surface of the polishing pad 32. As a result, a pressure difference is produced due to a force (pressure) required for the deformation of the semiconductor substrate 33, resulting in uneven pressing. Consequently, the problem of the semiconductor substrate 33 pressed unevenly remains unsolved.